<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
     .left{
         width:100px;
         height:100px;      
     }
     .nav{
         background-color: blue;
     }
     .contain{
         background-color: coral;
     }
    </style>
</head>
<body>
    <div class="left nav">

    </div>
    <div class="left contain">

    </div>
</body>
</html>